Preheat oven to 400°F. On a baking sheet, toss sliced mushrooms and onion in olive oil, salt and pepper. Roast the veggies for 20 to 25 minutes, until lightly browning around the edges.

In a non-stick skillet, melt the butter. Take the four slices and dip them in the melted butter. Be sure that the butter is evenly coating the four sides. Remove the skillet from the heat. Assemble the sandwiches in skillet in this order- bread (butter side down), 1/4 of the cheese, roasted onions and mushrooms, 1/4 of the cheese, bread (butter side up). Lightly press together to help things settle. Grill over medium heat until each side is lightly browned and the cheese is melty. Inspired by this recipe.